Many non-traditional congregations have modified the Avot prayer opening the Amidah, to add the Immahot. For example:
(Form Shabbat Siddur of the First Reform Minyan in UC Berkeley 2003)
What is the halachah on this?
The Talmud says that "we may not alter the coin that the Sages have minted -- En lecha reshut l'hosif 'al matbea' shetav'u chachamim bivrachot" [Berachot Y 86a]. This means that the prayers and blessings in the siddur may not be changed. That way, you can pray in any synagogue in the world and you will use the exact same words, and feel at home. That makes us one people.
Yet obviously the siddur has grown over the centuries. The Talmud does not forbid adding new prayers. We have seen the addition of many favorites, such as:
Lecha dodi -- 16th cent.; Shalom Alechem -- 15th cent.; Yigdal -- 15th cent.; Adon Olam -- 11th cent.; Prayer for Israel -- 1948; En kelohenu -- 9th cent.; kEl Adon -- 8th cent.; Hamavdil -- 11th cent.; Maoz Tzur -- 13th cent.; Yedid nefesh -- 16th cent.;
So, can we conclude that we may add a completely separate prayer extolling women (if that's what a congregation wants) without breaking halacha, but not make it part of the Amidah, which must remain in fixed form?